The use of a combustion reaction to synthesize a refractory material was first considered by Walton et al. [J. Am. Ceram. Soc., 42(1): 40-49 (1959)] who produced a composite ceramic/metallic material using thermite reactions. In the late 1960's, A. G. Merzhanov and his colleagues began work on self-propagating combustion reactions which led to the development of a process which they called "self-propagating high temperature synthesis" (SHS). [See Merzhanov et al., Dokl. Chem., 204 (2): 429-32 (1972); Crider, Ceram. Eng. Sci. Proc., 3 (9-10): 538-554 (1982).]
Self-propagating high temperature synthesis (SHS), alternatively and more simply termed combustion synthesis, is an efficient and economical process of producing refractory materials. [See for general background on combustion synthesis reactions: Holt, MRS Bulletin, pp. 60-64 (Oct. 1/Nov. 15, 1987): and Munir, Am. Ceram. Bulletin, 67 (2): 342-349 (Feb. 1988).] In combustion synthesis processes, materials having sufficiently high heats of formation are synthesized in a combustion wave which, after ignition, spontaneously propagates throughout the reactants converting them into products. The combustion reaction is initiated by either heating a small region of the starting materials to ignition temperature whereupon the combustion wave advances throughout the materials, or by bringing the entire compact of starting materials up to the ignition temperature whereupon combustion occurs simultaneously throughout the sample in a thermal explosion.
In the synthesis of refractory materials by conventional methods, the chemical reaction is initiated and carried to completion by heat from an external source such as a furnace. Usually, the heating rate is purposely kept low to avoid large temperature excursions caused by the high heats of reaction. Refractory materials prepared by such conventional methods are relatively expensive due to the high cost of energy and equipment. In the combustion synthesis process, however, after ignition has occurred, the rest of the sample is subsequently heated by the heat liberated by the reaction without the input of further energy. As a result, the power needed is much lower, and expensive equipment, such as high temperature furnaces, are not required.
Work on ceramic-metal composites has established that optimum physical properties are found in composites which have very small ceramic grains that are well dispersed within the metallic matrix materials. Conventional methods of synthesizing these composite materials leads to dense but typically large grained materials. Excessive grain growth occurring during conventional (non-SHS) synthesis leads to decreased strength of materials because of the association of large grain size with transgranular failure of the materials. Because of the very high heating and cooling rates and short reaction times of combustion synthesis, grain growth is slight, and therefore, unlike the products of conventional processes, the products of combustion synthesis are fine grained.
Advantages of combustion synthesis include: (1) higher purity of products; (2) low energy requirements; and (3) relative simplicity of the process. [Munir, supra at 342.] However, one of the major problems of combustion synthesis is that the products are "generally porous, with a sponge-like appearance." [Yamada et al., Am. Ceram. Soc., 64 (2): 319-321 at 319 (Feb. 1985).] The porosity is caused by three basic factors: (1) the molar volume change inherent in the combustion synthesis reaction; (2) the porosity present in the unreacted sample; and (3) adsorbed gases which are present on the reactant powders.
Because of the porosity of the products of combustion synthesis, the majority of the materials produced are used in powder form. If dense materials are desired, the powders then must undergo some type of densification process, such as sintering or hot pressing. The ideal production process for producing dense SHS materials would combine the synthesis and densification steps into a one-step process. To achieve the goal of the simultaneous synthesis and densification of materials, three approaches have been used: (1) the simultaneous synthesis and sintering of the product; (2) the application of pressure during (or shortly after) the passage of the combustion front; and (3) the use of a liquid phase in the combustion process to promote the formation of dense bodies. [Munir, supra at 347.]
Various methods of applying pressure have been incorporated into experimental SHS processes. Rice et al. [Ceram. Eng. and Sci. Proc. 7 (7-8): 651-760 (1968)] used a rolling mill technique on the systems, TiB.sub.2 --Al.sub.2 O.sub.3, TiC-Ti, and TiB.sub.2 -TiC. Miyamoto et al. [Comm. Am. Ceram. Soc., C-224-225 (Nov. 1984)], Yamada et al. [Am. Ceram. Soc. Bull., 64 (2): 319-321 (1985)], and Yamada et al. [J. Am. Ceram. Soc., 70 (9): C-206-C-208 (1987)] used a high pressure (3 giga Pascals) cubic anvil apparatus in a process which they call "high-pressure self-combustion sintering (HPCS)" to densify a variety of ceramic materials. Both Holt, supra and Takano et al. [Proc. 3rd Internatl. Conf. on Isostatic Pressing, Vol. 1, pp. 21-1 to 21-11 (London Nov. 10-12, 1986)] applied hot isostatic pressing (HIP) technology to the combustion synthesis of ceramic materials. The simplest method of applying pressure to a combustion synthesis reaction is the use of a hot pressing apparatus. That technique has been used by Holt et al. [J. Mat. Sci., 21: 251-259 (1986)]., Richardson et al. [Proc. 10th Ann. Conf. on Composites and Advanced Ceramic Materials, 7 (7-8): 760-770 (Fla. Jan. 19-24, 1986)], and Riley et al. [DARPA/Army SHS Symposium Proc., MTL SP 87-9: 153-166 (Fla. Oct. 21-23, 1985; eds. Gabriel et al.) and Army Ballistic Research Laboratories, BRL-MR-35-74 (Mar. 1987)], [See also: Soviet Patent No. 584,052 (Merzhanov et al. 1977); U.S. Pat. No. 4,431,448 (Merzhanov et al. 1984); U.S. Pat. No. 3,353,954 (Williams et al. 1967); (Holt et al, DARPA/Army SHS Symposium Proc., (Fla. Oct. 21-23, 1985) and UCRL-93467 (Jan. 1986); and Stringer et al., Proc. 4th Symp. on Spec. Ceram. (ed. Popper), 4: 37-55 (1967).]
Riley et al., supra, reports the combustion synthesis of ceramic/metallic composites (TiC and TiB2 with 10% Ni or Cu) wherein external pressures of up to 60 k psi were applied.
Borovinskaya et al. [Combust. Processes in Chem Tech. and Metallurgy, 141-146 (Moscow 1975)] investigated the interaction of Mo and Re with TiC formed during combustion synthesis in a high-pressure apparatus (200 atm). The product was 85% dense. The Mo and Re did not form an intermetallic compound but were alloyed in the process.
The present invention solves the problem of porosity of combustion synthesis products by applying relatively low pressure to the materials during or immediately following the combustion reaction. In doing so, the invention provides a low cost, commerically adaptable combustion synthesis process wherein synthesis and densification occur in essentially one step.
It is an object of this invention to produce materials by combustion synthesis that are less expensive than and have superior characteristics to those produced by conventional (non-SHS) processes. The fine grained and dense materials produced by the processes of this invention have enhanced fracture and impact strength as well as enhanced fracture toughness. For example, the invention provides alternative materials to those based on tungsten (W), which is very expensive due to its scarcity. The high hardness and melting point of titanium carbide (TiC) exceed those of tungsten carbide (WC); however, metallic composites of TiC produced by conventional processes have not gained commercial acceptance due mainly to their strength being lower than WC-Co. The comparatively low strength of such TiC composites can be attributed to excessive grain growth occurring during its formation in conventional processes. This invention solves that problem by providing a relatively inexpensive means to produce fine grained ceramic/intermetallic products that are also dense.
It is a further object of the invention to provide ceramic/intermetallic and ceramic/metallic composite materials wherein the ceramic grains are not only small in diameter but also spherical in shape and homogeneously dispersed within the intermetallic matrix. The materials produced according to the invention are improved by the sphericality of the ceramic grains in that the absence of angles removes potential stress points that could be sites of fracture or failure.